Resistive touch screens are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers.
FIG. 3 shows a portion of a prior art resistive touch screen 10 of the type shown in Published US Patent Application No. 2002/0094660A1, filed by Getz et al., Sep. 17, 2001, and published Jul. 18, 2002, which includes a rigid transparent substrate 12, having a first conductive layer 14. A flexible transparent cover sheet 16 includes a second conductive layer 18 that is physically separated from the first conductive layer 14 by spacer dots 20 formed on the second conductive layer 18 by screen printing.
Referring to FIG. 4, when the flexible transparent cover sheet 16 is deformed, for example by finger pressure, to cause the first and second conductive layers to come into electrical contact, a voltage applied across the conductive layers 14 and 18 results in a flow of current proportional to the location of the contact. The conductive layers 14 and 18 have a resistance selected to optimize power usage and position sensing accuracy. The magnitude of this current is measured through connectors (not shown) connected to metal conductive patterns (not shown) formed on the edges of conductive layers 18 and 14 to locate the position of the deforming object.
Alternatively, it is known to form the spacer dots 20 for example by spraying through a mask, or pneumatically sputtering small diameter transparent glass or polymer particles, as described in U.S. Pat. No. 5,062,198 issued to Sun, Nov. 5, 1991. The transparent glass or polymer particles are typically 45 microns in diameter or less and mixed with a transparent polymer adhesive in a volatile solvent before application. This process is relatively complex and expensive and the use of an additional material such as an adhesive can be expected to diminish the clarity of the touch screen. Such prior art spacer dots are limited in material selections to polymers that can be manufactured into small beads or UV coated from monomers.
It is also known to use photolithography to form the spacer dots 20. In these prior art methods, the spacer dots may come loose and move around within the device, thereby causing unintended or inconsistent actuations. Furthermore, contact between the conductive layers 14 and 18 is not possible where the spacer dots are located, thereby reducing the accuracy of the touch screen, and stress at the locations of the spacer dots can cause device failure after a number of actuations. Unless steps are taken to adjust the index of refraction of the spacer dots, they can also be visible to a user, thereby reducing the quality of a display located behind the touch screen.
Flexible touch screen devices using flexible substrates and circuitry are referenced in the art and described, for example in WO99/63792 entitled “Flexible Circuit Assembly” published Dec. 9, 1999. This disclosure describes a completely flexible circuit including a flexible substrate having a conductive coating deposited on it. The flexible circuit also includes a plurality of components fixedly attached to the flexible substrate and electrically interconnected with the conductive material. However, the use of a flexible substrate inhibits the use of conventional photolithographic techniques and flexing the substrate stresses the spacer dots and can cause them to come loose, thereby making the touch screen more likely to produce false positive signals.
There is a need therefore for an improved means to separate the conductive layers of a flexible touch screen and a method of making the same that improves the robustness of the flexible touch screen and reduces the cost of manufacture.